We're Constantly Bombarded By Radiation, But Don't Worry

May 25, 1986|By Don Colburn, Washington Post

Radiation is silent, invisible, odorless -- and everywhere.

Every person on Earth is constantly bombarded with low levels of natural radiation from cosmic rays and tiny amounts of radioactive elements in soil, water, rock, food and the human body itself. But the level of this ''background'' radiation -- about 100 millirems a year -- is so low that its effect on human health cannot be measured.

At very high doses, such as apparently occurred last month at the Chernobyl nuclear power plant in the Soviet Union, radiation can be lethal. But at low doses, such as from background sources or the fallout in the United States from the Chernobyl accident, radiation is much less harmful.

All radiation has some effect on the body. An expert panel of the National Academy of Sciences concluded in 1980 that there is no threshold below which radiation has no adverse effect on human health. But the exact nature and extent of the effect of low-level radiation is still unknown.

''The main thing for the public to understand is that with radiation, it's not all or nothing,'' said Dr. Niel Wald, chairman of the department of radiation health at the University of Pittsburgh School of Public Health. ''Many people think wrongly that if it's radiation, you're harmed; and if it's not, you're not.''

Normal background radiation comes from three sources:

-- Cosmic rays and other high-energy radiation from the sun and outer space. A five-hour transcontinental flight at 39,000 feet, above the protective layer of the Earth's atmosphere, exposes a passenger to about 2.5 millirems of cosmic radiation.

-- Terrestrial radiation from naturally radioactive elements, including radium and uranium in soil and rocks.

-- Natural radioactive substances in all living things, including food and human tissues.

An American is exposed to an average of 77 millirems a year of natural radiation -- 53 millirems from cosmic and terrestrial radiation and 24 millirems from radioactive materials in food and the body itself.

Medical and dental X-rays add an average of 50 to 100 millirems per year for Americans. Exposure from a chest X-ray, for example, averages about 15 millirems.

Other non-natural sources -- mostly fallout from above-ground nuclear bomb tests before 1963 -- account for less than 10 millirems of exposure per year. These sources include tiny amounts of radiation from nuclear power plants and even tinier amounts from color television sets, electron microscopes, airport inspection machines and glow-in-the-dark watches.

Background radiation is estimated by the National Academy of Sciences to cause about 1 percent of all cancers. But whatever cancers are caused by background radiation are statistically ''lost'' in the vast numbers of cancers that occur each year and can't be distinguished from other cancers.

There are two main kinds of radiation: non-ionizing, such as microwaves and radio waves, and ionizing, which comes from radioactive materials or X-ray machines.

Ionizing radiation alters normal atoms by knocking out one or more electrons to create charged atoms called ions. These highly unstable ions can damage human cells.

Even if it doesn't kill cells, ionizing radiation can short-circuit their information systems, damaging their ability to grow or reproduce. If the damage is too great for the body to repair, it causes radiation sickness or, sometimes years later, cancer.

Radioactive substances emit radiation in several forms, known as alpha, beta or gamma rays and X-rays. Gamma rays and X-rays penetrate the body easily; alpha and beta rays don't, but they can damage internal organs if ingested or inhaled.

Evidence from studies on animals suggests that a high dose of radiation in a short period of time has more effect than a lower dose given over a long period of time. For example, a person exposed to 100 rads from a bomb blast or nuclear power plant meltdown is more likely to develop radiation sickness than a person exposed to 10 rads per year for 10 years.

It could be compared to the difference between getting hit once by a speeding bullet and being repeatedly pelted with hand-tossed BB shot.